An evaluation of the BALROG and RoboBA algorithms for determining the position of Fermi/GBM GRBs

TL;DR

This study compares the accuracy of RoboBA and BALROG algorithms in localizing Fermi/GBM gamma-ray bursts, finding RoboBA generally provides more reliable uncertainty estimates, especially for faint bursts.

Contribution

It provides a comparative evaluation of RoboBA and BALROG algorithms for GRB localization, highlighting RoboBA's superior performance for faint bursts and better uncertainty calibration.

Findings

RoboBA outperforms BALROG for faint bursts.

RoboBA's uncertainties align well with actual errors.

BALROG underestimates uncertainties by about a factor of two.

Abstract

The Fermi/GBM instrument is a vital source of detections of gamma-ray bursts and has an increasingly important role to play in understanding gravitational-wave transients. In both cases, its impact is increased by accurate positions with reliable uncertainties. We evaluate the RoboBA and BALROG algorithms for determining the position of gamma-ray bursts detected by the Fermi/GBM instrument. We construct a sample of 54 bursts with detections both by Swift/BAT and by Fermi/GBM. We then compare the positions predicted by RoboBA and BALROG with the positions measured by BAT, which we can assume to be the true position. We find that RoboBA and BALROG are similarly precise for bright bursts whose uncertainties are dominated by systematic errors, but RoboBA performs better for faint bursts whose uncertainties are dominated by statistical noise. We further find that the uncertainties in the positions predicted by RoboBA are consistent with the distribution of position errors, whereas BALROG seems to be underestimating the uncertainties by a factor of about two. Additionally, we consider the implications of these results for the follow-up of the optical afterglows of Fermi/GBM bursts. In particular, for the DDOTI wide-field imager we conclude that a single pointing is best. Our sample would allow a similar study to be carried out for other telescopes.